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Three-Dimensional Nitrogen Doped Graphene/MnO Nanoparticle Hybrids as High-Performance Catalyst for Oxygen Reduction Reaction Ruwen Chen, Jing Yan, Yang Liu, and Jinghong Li J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b00306 • Publication Date (Web): 24 Mar 2015 Downloaded from http://pubs.acs.org on March 30, 2015

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Three-Dimensional

Nitrogen

Doped

Graphene/MnO

Nanoparticle Hybrids as High-Performance Catalyst for Oxygen Reduction Reaction

Ruwen Chen,†‡ Jing Yan, † Yang Liu,† Jinghong Li†* Department of Chemistry, Beijing Key Laboratory for Microanalytical Methods and

†

Instrumentation, Tsinghua University, Beijing 100084, China College of Environmental and Chemical Engineering, Nanchang Hangkong

‡

University of China, Nanchang 330063, China

*

Corresponding author. E-mail: [email protected]

1

ACS Paragon Plus Environment


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ABSTRACT Chemical doping with foreign atoms is an effective method to intrinsically modify the electronic properties of graphene. Besides, the hydration with metal oxide particles toward oxygen reduction with high activity can further improve its electro-catalysis performance. Furthermore, hierarchical structure can provide sufficient pathways to certify the diffusion of electrolyte and electron transfer. In this paper, we developed a novel three-dimensional nitrogen doped reduced graphene oxide/Manganese monoxide composite (3D-N-RGO/MnO) by incorporating covalent assembly and nitrogen doping. The as prepared 3D-N-RGO/MnO was further applied for oxygen reduction reaction (ORR). By the synergistic effect of three-dimensional nitrogen doped graphene (3D-N-RGO) and MnO, catalytic performance brings enhanced catalytic current and more positive potential. In addition, 3D-N-RGO/MnO exhibit excellent methanol tolerance and long-term stability.

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1. INTRODUCTION The oxygen reduction reaction (ORR) is of great significant for the development of many research fields involving fuel cells, metal-air batteries and the construction of biosensors.1-3 The rapid development of these applications calls for increasing demands for the improvement of ORR. Noble metal-based electrocatalysts have been long regarded as most efficient and active materials for ORR. However, not only the sluggish ORR kinetics but also the crossover effects, CO poisoning, poor stability in long-term operation and high price restricted its utilization. Furthermore, the research focused on finding cost-effective none-noble metal electrocatalysts alternation has attracted tons of interest.4, 5 In spite of the enormous ongoing researches for ORR electrocatalysts, it is still of great challenge to develop electrode catalysts with high activity. Graphene, a carbonaceous two-dimensional nanomaterial was seen widely used due to its excellent electronic and mechanical properties.6-9 The unique sp2-hybridized carbon network and high surface-to-volume ratio ensure its potential application as catalysts, which makes it an ideal candidate for ORR. In addition, the electron transfer rate can be significantly enhanced with hierarchical structures, foreign atom doping and formation of nanohybrid with metal oxide particles. Hierarchical structure can provide sufficient pathways to certify the diffusion of electrolyte and electron transfer.10-14 Modification of graphene on its backbone with the introduction of foreign atoms is possible to enhance its electrochemical behaviors with more active reactive sites.15-21 And the hydration with metal oxide particles toward oxygen reduction with high activity can further improve its electro-catalysis performance.22,

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Thus, the cost-effective material is more available compared to

Pt-based ORR catalyst. Manganese oxide, on the other hand, is another promising electrode material owing to its outstanding activity towards oxygen reduction. The catalytic activity of MnOx has been regarded from the redox between Mn species which assist the charge transfer to adsorbed oxygen on the catalyst surface. However, the low conductivity of 3



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pristine MnOx greatly limits the performance of oxygen reduction. Loading of MnOx on carbon materials carriers can improve the electrical conductivity and the ORR activity.24 The synergistic effect which enhances the charge absorb and delivers efficiency in the electrolyte solution attracts many researchers to explore superior high-performance ORR catalyst based on manganese oxide.25 In addition, MnOx was usually obtained by high-temperature calcination, and the synthesis of MnO was much simple. Herein, we propose a method to synthesize three-dimensional nitrogen doped reduced graphene oxide/MnO nanoparticle hybrid (3D-N-RGO/MnO) through covalent assembly and non-covalent metal oxide loading. Covalent assembly through glutaraldehyde and resorcinol aided cross linking was used to fabricate the three dimension graphene oxide network (3D-GO). The captioned Mn2+ was absorbed onto the GO layer by electrostatic adsorption to occupy abundant sites. The structure of 3D-GO was achieved through the lyophilization process, which also ensured the Mn2+ ions homogeneously distributed on the graphene network surface. The pyrolysis reaction afterward further reduced GO into nitrogen doped RGO and transferred the Mn2+ ions into MnO. Such architecture considerably improves its oxygen reduction activity based on shorter diffusion distance for better charge transport rate, larger surface to volume area of three-dimensional nitrogen doped graphene (3D-N-RGO) and high charge deliver efficiency of manganese dioxide. Compared to both oxide particle and graphene, the nanohybrids have a higher stability and increased electrocatalytic activity for oxygen reduction reaction, indicating potential application in fuel cell26 and biosensor application27. The process of fabricating 3D-N-RGO/MnO contains mainly three steps: (i) Mn2+ was anchored on to the graphene oxide surface by electrostatic interaction; (ii) three dimensional graphene oxide network (3D-GO) was formed by covalent assembly through glutaraldehyde and resorcinol aided cross linking; and (iii) pyrolysis reaction with urea to fabricate reduced graphene oxide with nitrogen doping and manganese oxide. In the experimental procedure, the Mn2+ ions were mixed with negatively charged GO suspensions under vigorous stirring. 4


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The 3D-GO structure was synthesised taking advantage of the hydroxyl groups on GO surface by covalent conjugation with glutaraldehyde. Furthermore, resorcinol improved mechanical stability for the sake of initiating polycondensation reaction. After freeze drying in lyophilizer, the 3D-GO/Mn(NO3)2 was obtained. Owing to the strong electrostatic attraction, they were bonded tightly. When the homogeneous mixture was ground with agate mortar and pyrolyzed at high temperature (900 ℃), three-dimensional nitrogen doped graphene/MnO (3D-N-RGO/MnO) was formed.

2 EXPERIMENTAL SECTION 2.1 Chemicals and Materials For 3D-N-RGO/MnO synthesis, graphite powder (99.99%, 325 mesh) was purchased from Alfa Aesar. H2SO4, K2S2O8, H2O2 (30%), KMnO4, Mn(NO3)2, P2O5, HCl, urea, resorcinol, glutaraldehyde, Na2B4O7·10H2O, K4[Fe(CN)6], K3[Fe(CN)6], KCl and KOH were obtained from Beijing Chemical Company. All reagents were analytically pure and used without further purification. 2.2 Synthesis of Three-dimensional Nitrogen Doped Graphene/MnO Nanoparticle Hybrids (3D-N-RGO/MnO) Graphene oxide (GO) was fabricated according to the previously reported Hummers’ method.28 Three-dimensional structure was fabricated by covalently interconnected according to the previously reported.29 In brief, 5 mg/mL GO dispersion was mixed with 22 mM glutaraldehyde, 0.06 mM borax, 11 mM resorcinol and 5 mg/mL Mn(NO3)2 under rigorous string. Then, the mixture turned viscous. After ultra sonication for 2 h the reaction was finished. After freeze drying for 48 h, 3D-GO/Mn(NO3)2 was formed. If not adding Mn(NO3)2, 3D-GO was obtained. Reduced 3D graphene oxide (3D-RGO) was obtained followed by pyrolysis at 900 ℃ under nitrogen atmosphere. The mixture of 100 mg 3D-GO and 500 mg urea was ground with a agate mortar and then pyrolyzed at 900 ℃ for 3 h. Three-dimensional nitrogen doped graphene (3D-N-RGO) was consequently obtained. The mixture 100 mg 3D-GO/Mn(NO3)2 and 500 mg urea was then pyrolysis at 900℃ 5



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under nitrogen atmosphere to get three-dimensional nitrogen doped graphene/MnO (3D-N-RGO/MnO) nanoparticle hybrids. 2.3 Characterization We utilized transmission electron microscopy (TEM, H-7650B) and scanning electron microscopy (SEM, JEOL JSM 7401) to characterize the surface morphology of as prepared material, respectively. The 3D-N-RGO/MnO was further identified by high-resolution transmission electron microscopy (HRTEM, JEOL, JEM-2010). Energy-dispersive X-ray spectrum (EDS) was analysed by energy dispersive spectroscopy analyser attached on the HRTEM. The crystallographic structures of the samples were identified by powder X-ray diffraction (XRD) with a Bruker D8-Advance. Raman spectra were employed to characterize the structural information of the materials. XPS spectrum was employed to characterize the element content of 3D-N-RGO/MnO. 2.4 Electrochemical Investigation Electrochemical measurements were carried out using a conventional three-electrode system composed of a glassy carbon working electrode (GCE), a Pt wire counter electrode, and a KCl saturated Ag |AgCl reference electrode. Ahead of usage, the GCE was polished with α-Al2O3 slurry on an abrasive cloth, and rinsed with water, ethanol, and water respectively under ultrasonication, the electrode was dried with high-purity nitrogen afterwards. We prepared the samples by drop-casting 5 μL of 1 mg/mL suspension over the GCE and evaporating the residual water at room temperature. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were performed with a CHI 1030B Instrument (CH Instruments, Inc., USA) and a PARSTAT 2273 advanced electrochemical system (Princeton Applied Research, USA), respectively.

Additionally, the EIS

measurements were tested by applying an AC voltage of 10 mV amplitude with frequency from 100 Hz to 100 kHz in 5 mM K3[Fe(CN)6]/K4[Fe(CN)6] (1:1) redox probe solution.

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3. RESULTS AND DISCUSSION As prepared 3D-N-RGO/MnO was characterized by scanning electron microscopy (Figure 1A and B), transmission electron microscopy (Figure 1C) and high resolution transmission electron microscopy (Figure 1D, E) to reveal the microscopic structures of the nanohybrids. The porous network structure is easy to be observed in Figure 1B. On the surface of three-dimensional network formed by stacked graphene sheet, homogeneously dispersed MnO particles can be clearly seen, the size range of which was estimated from 40 to 50 nm (Figure 1C). The successful cross-linking between single building blocks under the glutaraldehyde aided reaction can be verified by the wrinkles in the TEM images. Even though the three-dimensional structure was highly interconnected, 3D-N-RGO/MnO was still well dispersed without aggregations. The apparent lattice fringe of MnO particles indicated high crystallinity after pyrolysis. The lattice spacing was measured to be 0.25 nm, which can be indexed to (111) crystal planes of MnO (Figure 1E). Compared with the 3D-N-RGO/MnO, pure MnO particles (Figure 1F) exhibit irregularly shape and the aggregation is serious. This result suggested that using of the growth site of graphite oxide surface can get regularly shaped and good dispersity of MnO. X-ray diffraction (XRD) was used to characterize the crystallographic structures of 3D-N-RGO/MnO, 3D-N-RGO, 3D-RGO and MnO. The typical strong peak near 25° (Figure 2A) reveals the efficient reduction of graphene oxide. The XRD pattern of MnO was identical to that of manganosite (JCPDS card No. 75-1090). There was no other characteristic peak appeared in the pattern of 3D-N-RGO/MnO, suggesting that the addition of RGO didn’t affect the formation of MnO. Raman spectroscopy was employed to characterize the structural information of the carbonaceous materials. The ID/IG values of 3D-N-RGO/MnO, 3D-N-RGO, 3D-RGO and GO were 1.14, 1.08, 1.22 and 0.91 respectively. The higher ID/IG value of 3D-RGO (1.22) than that of 3D-GO (0.91) verified the reduction of 3D-GO during the hydrolysis. Lower ID/IG value (1.08) of 3D-N-RGO compared to 3D-RGO (1.22) was attributed to the 7



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electron-donating capability of N heteroatom induced graphitic degree decrease. The change of ID/IG value offers powerful evidence of the reduction and N-doping in GO. It is also noticed that from Raman spectra (Figure 2B) that the peaks at 1346.2 cm−1 and 1592.5 cm−1, corresponding to the characteristic D and G bands of graphene. The specific Raman vibrational modes located at 650.3 cm−1 and 366.3 cm−1 confirm that the attached MnO NPs were present. X-ray photoelectron spectroscopy (XPS) was utilized to investigate the chemical composition of nanosized 3D-N-RGO/MnO. As shown in Figure 2C, the C1s, N1s, O1s, Mn2p peaks clearly reveal the presence of graphene and manganese oxide. In the XPS spectrum of 3D-N-RGO, apparent N1s peak was clearly seen, suggesting the nitrogen atom has been doped into the graphene backbones. The binding energy values from the high-resolution XPS spectra (Figure 3D) can further show the binding state between carbon and nitrogen in 3D-N-RGO. Peak a was near 398.5 eV, corresponding to pyridinic N, peak b was near 399.8 eV, corresponding to pyrrolic N and peak c was near 401.1 eV, corresponding to quaternary N respectively. The Mn2p XPS spectra of the nanohybrid in Figure 2C exhibits two peaks at 641.5 eV and 633.4 eV, which are corresponding to 2p2/3 and 2p1/2 respectively. The mass percentages of nitrogen and MnO are estimated to be 5.70% and 24.4% separatively. The cyclic voltammograms (CVs) of ORR on the 3D-N-RGO/MnO, 3D-N-RGO, 3D-RGO and MnO in O2-saturated 0.1 M KOH solutions are shown in Figure 3A. The obvious increase of the peak current density from MnO, 3D-RGO, 3D-N-RGO, 3D-N-RGO/MnO in sequence was contributed to the multidimensional electron transfer pathways benefited from the 3D structure and the electroactive sites resulting from nitrogen doping and MnO composite. Pure MnO exhibited very poor ORR catalytic activity with the onset potential and reduction peak potential at around -0.27 and -0.41 V, respectively. 3D-RGO exhibited ORR catalytic activity with the onset potential and reduction peak potential at around -0.25 and -0.37 V, respectively. 3D-N-RGO showed similar cathodic peak current to 3D-RGO, but had a more positive onset potential at -0.22 V and peak potential at 0.36 V indicating better catalytic performances. The 3D-N-RGO/MnO showed a pronounced improvement for 8


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ORR-the onset potential and reduction peak potential shifted positively to around -0.15 and -0.35 V, respectively, with a concomitant increase in cathodic peak current, suggesting the synergistic ORR catalytic activity of 3D-N-RGO and MnO in the hybrid. The electron transfer ability of 3D-N-RGO/MnO was further investigated by electrochemical impedance spectroscopy. As shown in Figure 3B, the diameters of the semicircles of 3D-N-RGO/MnO, 3D-N-RGO, 3D-RGO and MnO electrodes in the characteristic impedance curves (Nyquist plots) in sequence are contributed to decrease. By fitting the results with an appropriate equivalent circuit, the charge transfer resistance values are calculated to be 135.10 Ω at MnO modified GCE, 32.91 Ω at 3D-RGO modified GCE, 14.48 Ω at 3D-N-RGO modified GCE and 2.74 Ω at 3D-N-RGO/MnO modified GCE. These results verify that the three-dimensional nitrogen doped reduced graphene oxide/Manganese monoxide composite greatly improved the electrochemical performance on the glassy carbon interface with fast electron transfer rate. The electrocatalytic performance of 3D-N-RGO/MnO for ORR was investigated by cyclic voltammograms (CVs) in O2-saturated, N2-saturated 0.1 M KOH solution and in O2-saturated 0.1 M KOH solution with 3 M methanol. As shown in Figure 4A, a well-defined cathode peak emerged at -0.351 V can be clearly seen in the CV curve, which is different from the indistinctive CV curve in N2 saturated solution, indicating the excellent oxygen reduction reaction of the nanohybrid. In Figure 4A, the CV curve was not affected by the addition of 3 M methanol. These results reveal the stability of the new kind of catalyst, making it a promising candidate for direct methanol

fuel

cell.

To

further

evaluate

the

catalytical

performance

of

3D-N-RGO/MnO, linear sweep voltammograms (LSVs) on a rotating disk electrode (RDE) were performed in O2 saturated 0.1 M KOH solutions at a scanning rate of 5 mV s-1. As can be seen in Figure 4B, compared to 3D-N-RGO/MnO, 3D-N-RGO, 3D-RGO and MnO, 3D-N-RGO/MnO showed the best ORR performances with the higher onset potential, an evident increase in limiting diffusion current at -0.5 V, and a relatively flat and wide current plateau. When the rotating rate changed from 625 to 9



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2500 rpm, the voltammetric current of the 3D-N-RGO/MnO electrode increased with the square root of scan rate linearly (Figure 4C). And the electron transfer number (n) was calculated to be 3.03 according to Kouteky-Levich (K-L) plots (Figure 4D), which surpass the current density and electron transfer number of graphene and MnO. The Kouteky-Levich (K-L) equations are listed below. 1 1 1 1 1     1/ 2 J J L J K B JK B  0.62 nFC 0 ( D 0 ) 2 / 3 v  1/ 6 J K  nFkC 0

(1) (2)

Where J is the measured current density, JK and JL are the kinetic and diffusion limiting current densities, ω is the electrode rotating rate (ω = 2πN, N is the linear rotation speed), n is the overall number of electrons transferred in the oxygen reduction, F is the Faraday constant (96485 C mol-1), C0 is the bulk concentration of O2, D0 is the diffusion coefficient of O2 in the KOH electrolyte, v is the kinematics viscosity of the electrolyte, and k is the electron transfer rate constant. The electron transfer number illustrates that the ORR of 3D-N-RGO/MnO was a combination of two-electron and four-electron reaction pathways. Such nanostructured carbonaceous nanomaterials with MnO nanoparticles enhanced the electrocatalytic activity towards oxygen reduction with abundant active reaction sites. The nitrogen doping site can serve as active catalytic center for ORR. It’s essential to estimate the selectivity and stability of a new kind of electrocatalytic material by chronoamperometric measurements. The relative current of 3D-N-RGO/MnO maintained to be 64.9% after 20000 s continuous reaction (Figure 5). The good stability was owing to the stable three-dimensional composite structure of the material. The curves presented some small fluctuations, which is caused by the fluctuation flow.

4. CONCLUSIONS In conclusion, we have constructed a three-dimensional nitrogen doped graphene nanostructure with MnO loading. The new system was utilized to catalyse oxygen 10


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reduction reaction. It shows good catalytically activity, excellent stability and resistance, which make it an ideal candidate metal-free ORR catalyst. Furthermore, the distinctive structure opens novel possibilities to promote graphene/metal oxide composition in the application of catalyst, fuel cells, efficient energy-conversion and biosensor systems.

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (No. 21235004), National Basic Research Program of China (No. 2011CB935704, No. 2013CB934004).

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Ozden, S.; Patra, P. K.; Pasquali, M.; Vajtai, R.; Ganguli, S. Covalently Interconnected Three-Dimensional Graphene Oxide Solids. ACS Nano 2013, 7, 7034-7040.

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Captions Figure 1. SEM (A, B), TEM (C), HRTEM (D, E) images of 3D-N-RGO/MnO and TEM (F) image of pure MnO.

Figure 2. (A) X-ray diffraction patterns and (B) Raman spectra of 3D-N-RGO/MnO, 3D-N-RGO, 3D-RGO and MnO; (C) XPS survey spectra of 3D-N-RGO/MnO; (D) High-resolution XPS spectrum of 3D-N-RGO/MnO showing N1s.

Figure 3. (A) CV curves of 3D-N-RGO/MnO, 3D-N-RGO, 3D-RGO and MnO in O2-saturated 0.1 M KOH solution. Scan rate: 50 mV s-1; (B) EIS of 3D-N-RGO/MnO, 3D-N-RGO, 3D-RGO and MnO in 5 mM Fe(CN)63-/4- containing 0.5 M KCl solution. The frequency range is from 100 mHz to 100 KHz with signal amplitude of 10 mV.

Figure 4. (A) CV curves of 3D-N-RGO/MnO in O2-saturated, N2-saturated 0.1 M KOH solution and in O2-saturated 0.1 M KOH solution with 3 M methanol. Scan rate: 50 mV s-1; (B) Linear sweep voltammetry curves of 3D-N-RGO/MnO, 3D-N-RGO, 3D-RGO and MnO in O2-saturated 0.1 M KOH solution at a rotation rate of 1600 rpm. Sweep rate: 5 mV s-1; (C) RDE curves of 3D-N-RGO/MnO in O2-saturated 0.1 M KOH solution at various rotation rates. Sweep rate: 5 mV s-1; (D) and corresponding K–L plots of 3D-N-RGO/MnO at different potentials.

Figure 5. The current-time (i–t) responses of 3D-N-RGO/MnO at -0.4 V in O2-saturated 0.1 M KOH solution with a rotation rate of 1600 rpm.

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Figure 1

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Figure 2

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Figure 3

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Figure 4

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Figure 5

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Table of Contents (TOC) Graphic

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MnO Nanoparticle

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